The present invention relates to apparatus for optically recording signals on recording mediums, and more particularly to an optical recording apparatus using a light beam for recording a signal on a recording medium as it passes accurately through a prescribed path in an optical system.
In the prior art, there is a system wherein a signal is recorded by forming pits, responsive to the light signal, along a spiral track or concentric circular tracks on a rotary recording medium. In this recording system, a light beam is modulated in response to the signal and projected onto a recording medium coated with a light-sensitive material such as photoresist. Thereafter, the disc is developed to provide an original recorded disc.
The shape of the projected spot of the light beam falling on the original disc is such that the dimension in the track extending direction is less than the dimension in the track width or transverse direction. On another hand, the cross-sectional shape of a light beam emitted from a light source (such as a laser light source) is substantially circular. Accordingly, in order to shape the emitted light beam into the desired spot shape, a cylindrical lens and, in addition, a slit plate having a long narrow slit are used in the optical system.
In general the distribution of the intensity of the light beam, is maximum at the center thereof and the spreading beam exhibits a Gaussian distribution. Consequently, if the center of the Gaussian distribution of a light beam passing through a slit in the slit plate is offset from the center of the slit, the light beam, which has passed through this slit has a limited width, and there is an asymmetrical light intensity distribution. If recording is carried out with a light beam of this character, the pits formed on the recording medium will also have an asymmetrical shape. When a recording medium having this asymmetrical pit formation is reproduced, a good reproduction cannot be carried out. Furthermore, other problems such as nonuniform wear of both the tracing reproducing stylus and the recording medium arise, which shortens their serviceable lives.
On the other hand, the optical system is so adjusted and set at the time of its assembly that the center of the light beam coincides with the center of the slit. However, there are a number of causes of inaccuracy such as, for example, variation due to, change of the emission direction of the laser beam from the laser light source, and nonuniform attenuation across the beam cross section as the laser beam passes through light modulators. Also, with the passage of time, there are extremely small displacements of the lens mounting and adjusting mechanism parts of the optical system. As a result of these and similar causes, even when the above mentioned adjustments and settings are accurately carried out at the time of assembly, the center of the Gaussian distribution of the light beam no longer coincides with the center of the slit during the lifetime of the recording apparatus.
In such a case, it has been the practice heretofore to manually readjust the optical system each time that there is a problem so that the two centers of the beam and slit coincide. This readjustment procedure is extremely troublesome.
Another problem arises during recording in the case where a main information signal and a reference (pilot) signal are projected simultaneously onto the recording medium. The pilot signal is for use in tracking control to insure proper stylus position with respect to the main information signal track at the time of reproducing. During this recording operation, the two recording beams must continually be precisely in a specific positional relationship with respect to each other. However, even if the optical systems of the two recording light beams are correctly adjusted and set, there is the same problem as that discussed above. That is, the problem is one of noncoincidence of the Gaussian distribution center of the light beam and the center of the slit or pin hole. This problem arises with respect to each of the two (information and pilot) optical systems.